Extend goinstall to support downloading from any hg/git/svn/bzr hosting
site, not just the standard ones.  The type of hosting is automatically
checked by trying all the tools, so the import statement looks like:

  import "example.com/mything"

Which will work for Mercurial (http), Subversion (http, svn), Git (http,
git) and Bazaar (http, bzr) hosting.

All the existing package imports will work through this new mechanism,
but the existing hard-coded host support is left in place to ensure
there is no change in behaviour.

R=golang-dev, bradfitz, fvbommel, go.peter.90, n13m3y3r, adg, duperray.olivier
CC=golang-dev
https://golang.org/cl/4650043